1. Field of the Invention
The present invention generally relates to semiconductor fabrication, and particularly to the technology of extreme ultraviolet (EUV) photomask.
2. Description of Related Art
In the fabrication process for the semiconductor device in electronic circuit, it usually includes photolithographic and etching processes, so to form the internal structure for the semiconductor device. The photolithographic process needs the light source to irradiate the photoresist and develop the image. The mask pattern is then obtained to serve as the etching mask in the subsequent etching process. However, as the need to increase device density, the size for the pattern structure is accordingly reduced. Therefore, the light source used in the photolithographic process should take the light with shorter wavelength to obtain the capability of higher resolution. Then, the fine structure of the pattern can be well developed with clarity.
The wavelength of EUV is about 13.5 nm. The EUV is the suitable light source in developing the photomask technology, so at satisfy the requirement of high resolution. As to the EUV photomask, usually, the pattern is formed on a reflection layer of high reflectivity, in which the pattern as an example is a light-absorbing material. When the EUV light source is incident onto the EUV photomask with an incident angle, a portion of the light source shooting on the reflection layer is reflected to the photoresist layer and another portion of the light source shooting on the light-absorbing material is absorbed without reflection to the photoresist layer. Thus, the reflection light carries the intended pattern and shoots onto the photoresist layer. The photoresist layer is developed with image and then gets the photoresist pattern, which can be used in the subsequent process, such as etching process or implanting process.
Since the light-absorbing pattern is protruding from the reflection layer and the light is incident with a tilt angle, the light-absorbing pattern would cause a shadow with respect to the incident light, which may affect the line width in an example. This is an issue needed to be concerned in designing the EUV photomask.